-48V Hot-Swap Controllers with External

_______________________________________________________________________________________

9

Board Insertion

Figure 6a shows a typical hot-swap circuit for -48V sys-

tems. When the circuit board first makes contact with

keep the gate-to-source voltage of Q1 low during hot

insertion preventing Q1 from passing an uncontrolled

current to the load. For most applications, the internal

MAX5939 eliminates the need for an external gate-to-

source capacitor. The resistor R3 limits the current into

the clamp circuitry during card insertion.

Power-Supply Ramping

The MAX5921/MAX5939 can reside either on the back-

plane or the removable circuit board (Figure 6a). Power

is delivered to the load by placing an external N-chan-

nel MOSFET pass transistor in the power-supply path.

After the circuit board is inserted into the backplane,

undervoltage and overvoltage tolerance, the

MAX5921/MAX5939 gradually turn on the external

MOSFET by charging the gate of Q1 with a 45µA cur-

rent source. Capacitor C2 provides a feedback signal

to accurately limit the inrush current.

The inrush current can be calculated:

Figure 6b shows the inrush current waveform. The cur-

rent through C2 controls the GATE voltage. At the end

of the DRAIN ramp, the GATE voltage is charged to its

final value. The GATE-to-SENSE clamp limits the maxi-

mum

Board Removal

If the circuit card is removed from the backplane, the volt-

age at the UV falls below the UVLO detect threshold, and

the MAX5921/MAX5939 turn off the external MOSFET.

Current Limit and Electronic Circuit

Breaker

The MAX5921/MAX5939 provide current-limiting and cir-

cuit-breaker features that protect against excessive load

current and short-circuit conditions. The load current is

monitored by sensing the voltage across an external

MAX5921

MAX5939

OV

UV

SENSE

GATE

DRAIN

PWRGD

-48V RTN

-48V RTN

SHORT PIN

VICOR

VI-J3D-CY

-48V

GATE IN

C4

100

µF

100V

C3

0.1

µF

100V

C2

15nF

100V

R3

1k

Ω

5%

R2

10

Ω

5%

Q1

IRF530

R1

0.02

Ω

5%

R4

549k

Ω

1%

R5

6.49k

Ω

1%

R6

10k

Ω

1%

4.7nF

Figure 6a. Inrush Control Circuitry/Typical Application Circuit